Process for preparing aromatic isocyanate

ABSTRACT

A one-step process for the industrial production of aromatic isocyanate in high yield is provided. Aromatic nitro compound is reacted with carbon monoxide at an elevated temperature under high pressure in the presence of solvent by using the catalyst of chlorides of palladium and/or rhodium or the complex thereof, and pyridine in the amount not less than 4 mols based on 1 mol of said metal element. The resulting product is useful intermediate for producing urethanes, carbamic acid derivatives and pharmaceuticals.

United States Patent [191 Mizoguchi et al. Sept. 2, 1975 [5, PROCESS FOR PREPARING AROMATIC 3,812,169 5/1974 Hammond et al. 260/453 XSOCYANATE [75] Inventors: Yoshiyuki Mizoguchi; Syohachi Primary Examiner-Elbert L. Roberts Ono; Toshiyuki Iwaisako, all of Assistant Examiner-Dolph H. Torrence Tokyo, Ja an Attorney, Agent, or Firm-Armstrong, Nikaido & [73] Assignee: Asahi Kasei Kogyo Kabushiki wegner Kaisha, Osaka, Japan [22] Filed: Dec. 27, 1973 [57] ABSTRACT PP N05 428,898 A one-step process for the industrial production of aromatic isocyanate in high yield is provided. Aromatic [30] Foreign Application Priority Data nitro compound is reacted with carbon monoxide at J 10 1973 J 48 5265 an elevated temperature under high pressure in the presence of solvent by using the catalyst of chlorides of palladium and/or rhodium or the complex thereof, 260/453 252/42: and pyridine in the amount not less than 4 mols based 58 d 260/453 PC on 1 mol of said metal element. The resulting product 1 o earc is useful intermediate for producing urethanes, car- I References Cited bamic acid derivatives and pharmaceuticals.

UNITED STATES PATENTS 4/1971 Smith et al 260/453 6 Claims, 1 Drawing Figure PATENTEDSEP 21% 3,903,125

molar correlation between pyridine and palladium Oo no phenol added phenol added IOO- L o o conversion of 2,4diniiroioluene yield(%) of Toluene K 2,4-diisocyanate (no phenol added) yield(%) of Toluene 2,4-diisocyanare (phenol added) reaction condiiion Temperature 2lOC CO pressure I20 kg/cm (room Temperalure) Time 2.0 Hrs i s lo 2'0 p /pd molar ralio PROCESS FOR PREPARING AROMATIC ISOCYANATE BACKGROUND OF THE INVENTION Lll'llC isocyanates are popular intermediates widely used in various commercial fields such as foams, adhesives, coverings, and fibers all as a form of urethanes; carbamic acid derivatives and pharmaceutical preparation.

As regards the production method of isocyanates, various processes are heretofore known. One of the conventionally established methods is so called aminephosgene which comprises preparing amine from corresponding nitro compounds of nitrile compounds and then subsequently reacting thus prepared amine with phosgene. However, this method includes many defects such as complexity of the process itself and the use of heavily toxic phosgene, and therefore improvement has been desired from the commercial view point.

In order to satisfy the commercial feasibility, a method in which organic nitro compound was reacted with carbon monoxide to obtain isocyanate was proposed. Namely, British Patent No. 1080094 disclosed a process in which urethane is prepared from nitro compound, carbon monoxide and alcohol in the presence of combination catalyst of palladium, iridium, rhodium or nickel compound, and heavy metal compounds at a temperature of 100 175C. Further, British Patent No. 1025436 discloses a process for preparing isocyanate by using chlorides of rhodium, palladium, iridium or osmium. DOS (German Patent Unexamined) Nos. 2005810, 2005811, 2005812 and 2011809 disclose a process wherein precious metal, amide, nitroso, nitrile and amino compound are added to the catalyst system. Japanese Patent Publication 5721/1968 discloses the catalyst of precious metal and Lewis acid compound. DOS. No. 2011810 aiming the enhancement of the yield discloses a process wherein isocyanate is prepared by adding alcohol in the presence of catalyst system of precious metal, Lewis base and the compound of Va and Vla group compound. In this DOS phenol as a alcohol is added in the presence of catalyst system of pyridine/palladium being 2 mol.

In all of the above mentioned methods, high pressure and a temperature of from 25C 250C are required as a reaction condition. In addition, due to the use of phenol, there occur many problems such as corrosion of the reactor, formation of carbamic acid by the reaction of phenol and isocyanate, recovery of phenol, etc. The most serious drawback of these known methods resides in the features oflow yield of the isocyanate which cannot justify the feasibility of the process for a commercial scale.

The present inventors made an extensive study on the catalyst system and the reaction conditions in order to solve the above mentioned problems and finally the present inventors reached the invention which attained high yield and high selectivity.

BRIEF SUMMARY OF THE INVENTION The present invention relates to an improved process in preparing aromatic isocyanate by catalytic reaction of aromatic nitro compound with carbon monoxide at an elevated temperature under high pressure, which process is characterized by effecting the reaction in the presence of solvent at a temperature of 200 250C by using the catalyst of chlorides of palladium and/or rhodium or the complex thereof, and pyridine in the amount of not less than 4 mols based on 1 mol of said metal element, and which process enables to obtain the isocyanate in a greatly enhanced yield. Due to the simplicity of the reaction system using nitro compound, carbon monoxide, solvent, metallic chloride and pyridine according to the present invention, various advantages such as to produce isocyanate in a very low cost, to avoid corrosion of the reactor at the time of the phosgene production, or to ensure no pollution processes, are attained according to the present invention, which advantages could never be attained according to the conventional phosgene production process by reacting amine compound with phosgene.

Comparing to the known processes as disclosed in the afore-said patents, the present invention shows 100% of nitro compound conversion and of yield of the corresponding isocyanate, the results of which are far superior to those of the known processes. Further, according to the present invention, the composition of the reaction product in the resultant reaction liquid is in the isocyanate and other unknown impurities, if present, do not adversely affect separation by distillation.

In the present invention, metallic chloride which greatly influences the cost of the isocyanate production forms complex with pyridine and thus resultant complex can be recovered in a form of needle-like crystal and can be repeatedly used by recycle.

Usually in order to effect the carbonylation reaction it is required to use, due to extreme corrosion, the reactor coated by silver, aluminum or copper. However, according to the present invention, it is possible to use a stainless steel reactor.

As mentioned above, the present invention possesses many advantages comparing to the known methods and enabled commercially feasible production of aromatic isocyanate.

BRIEF DESCRIPTION OF THE DRAWING The drawing shows the schematic result of Examples 5 8 in comparison with Control 1 7. The figure shows molar correlation between pyridine and palladium in terms of conversion and yield.

DETAILED DESCRIPTION OF THE INVENTION For the aromatic nitro compound employed in the present invention, any aromatic nitro compound used in the conventional processes can be employed as a starting material of the present invention. Namely, aromatic monoor poly-nitro compound or the substitutes thereof can give in high yield corresponding monoor poly-isocyanate by the reaction according to the present invention. Typical examples of aromatic nitro com-- pounds which can give isocyanate by the reaction with carbon monoxide are nitro benzene, nitro naphthalene, nitro anthracene, nitro biphenyl, bis(nitrophenyl) methane, bis(nitrophenyl) ethane, bis(nitrophenyl)ether, bis(nitrophenyl)sulfone, nitro phenotheazine, nitro phenoxyalkane and substituted compound of the said aromatic nitro compound which are substituted by at least one of nitro group, nitro alkyl group, isocyanate group, alkyl group, alkenyl group, alkoxy group and halogene. Further, it is also possible to use isomer or isomeric mixtures of said aromatic nitro compounds and substituted aromatic nitro compounds. Aromatic nitro compounds preferably employed for the present invention are monoor poly-nitrated nitro benzenes and isomeric mixtures thereof: nitro alkyl benzene and isomeric mixtures thereof; and bist'nitrophenyl) alltyl and'isomeric mixtures thereof 7 The catalyst system employed for the present invention is the mixture of pyridine and chloride of palladium orrhodium or the mixture of pyridine and the' complex of said metal. The catalyst may be prepared by separately adding metallic compound and pyridine to the reaction system, or by adding mixture thereofto the reaction system. Further, the complex prepared by the conventionally known method can be used as a metallic compound or pyridine. I

For the chlorides of palladium or rhodium, palladium dichloride, rhodium trichloride or the hydrate thereof treated in suitablemanner can be used. Further, the ox; ides or, thechlorides prepared from metal by a various method canbe used. Y i

The amount ofpyridine is especially important factor inthe present catalyst system and the presence of pyridinerequired is not less than 4 mols based on said metal. Pyridine contained in the complex can be taken into account for the amount of pyridine required, and usually the amount of 4 20 times of mols, particularly 4 9 times of mols, is preferable. Employment of the amount below not more than l times, results in remark able ,decrease particularly in isocyanate yield and therefore is not suitable for the present invention. For,

example, palladium complex containing 2 mols of pyridine [Pd(Py) Cl as catalyst, as starting nitro benzene asstarting material, and L1 ,2 trichlro-l ,2,2- trifluoroethane as solvent were charged in a 120 ml. of

stainless steel (SUS 32) reactor and the reaction was.

effected ata temperature of 190C and at a CO pressure of lZO Kg/cm (calculated by room temperature) for- 2 hours. The-result showed the conversion of nitro compound was 100% and the yield of phenyl isocyanate was as low as 33.7%. in this example only poor result was obtained, whereby coagulated by-product was adbored to the wall of a reactor. Proposed improvement (German Patent l i810) for solving this problem, wherein alcohols are added to the reaction system, still gives only a poor result of conversion-of nitro form being 96%, diisocyanate yield being 25.0% and the yield including monoisoc-yanate being 52.3%. Further, this improvement was unsatisfactory because the reaction liquid is viscous and corrosion of the reactor occurs.

he present invention will be further explained by the drawing of the invention. The drawing shows the results of Example 5 8 and Controls l 7. The abscissa shows the molar ratio of pyridine and palladium and the ordinate shows conversion of 2,4-dinitro toluene and the yield of toluene-2, d-diisocyanate.

The drawing clearly shows the difference in the result between the effects by addition and non-addition of alcohols (phenol). Nameiy, in case phenol is added, the highest yield can be obtained at a molar ratio of pyridine to palladium being 2:1. But, the range suitable for thepresent invention is not less than times of mols.

Namely, the catalyst system according to the present invention enables to attain higher yield (more than 70%) of organic isocyanate and to employ the simpli fied catalyst system, thereby making ready recovery of the catalyst. Further in the present invention. it is meaningless to add oxides of Group VB- and Vibmetals to the reaction of the present invention, because the addition merely causes decrease in yield, promo tion of by-rcactiom and complexity of catalyst recov ery.

benzene, dichlorobenzene. monochloro toluene, trichloro ethane, tetrachloro ethane, trichloro trifluoro ethane andperchloro ethylene; aromatic hydrocarbons such as benzene. toluene and xylene; and mixture, thereof. Particularly preferable solvents are aromatic hydrocarbon and halogenated hydrocarbon, for example mono and poly-chloro benzene, benzene, toluene;

etc.

As regards the ratio of solvent to starting aromatic nitro compound, any ratio can be employed as long as the facilities do not become too large.

Carbon monoxide used for the reaction is charged with the pressure of 40 200 Kg/crn into an autoclave. The reaction with a pressure not more than 40 Kg/cm necessitates higher temperature and longer reaction time for proceeding the reaction and therefore disadvantageous for the production of isocyanate. Furthena reaction with the pressure more than 200 Kg/cmh'.

though the reaction itself can take place, is not prefera+ ble because the use of highly pressure resistant reactor is necessitated.

in practice. according to the present catalyst system,

reaction canbe proceeded at a pressure below 200 g/cm Carbon monoxide can be charged continuously or intermittently as the reaction proceeds by in creasing the temperature.

The present reaction proceeds assumingly by the lowing reaction formula:

smo snco R(NCO),, znco The amount of carbon monoxide necessitated in the reactor room is 3 times of mols based on nitro group, and for practising the. invention it is preferable to charge 5 10 times of mols. There is no upper limit for the amount.

Carbon monoxide other than that consumed for the present reaction can be circulated by a suitable means for re-use. I

in the present invention, reaction-temperature is particularly important factor. Namely, the temperature below 200C promotes the side reaction, lowers the yield and selectivity, and accordingly tends to increase the reaction residue. (Confer Control 8). lncrease of the reaction residue is unfavorable, because the residue causes adhesion to the wall of the reactor and makes it.

impossible to conduct a continuous reaction, to recover the starting materials and to separate the product. Further, in the range above 250C, the purpose of the invention can not be attained. because of decomposition of the produced isocyanate, and decrease of yield and selectivity.

The suitablerange ol the reaction temperature varies according to the kind and the reactivity of aromatic nitro compounds tobe employed, each of which has different reactivity and the produced corresponding isocyanates having different thermal resistance, and usually a temperature of 200 250C, preferably 200 240C is employed with the use of exterior or interior heating apparatus. I

The reaction time varies with the kind of aromatic nitro compound to be employed for the reaction, the

EXAMPLE 2 The same procedure as in Example 1 was followed,

' except that 10.9 g. of 2,4-dinitro toluene, 1.6 g. of palof the product was calculated by gaschromatography analysis using biphenyl as an internal standard. The conversion of nitrobenzene was 100% and the yield of the phenyl isocyanate was 78.1%.

catalyst system and the amount thereof, solvent and ef- 5 ladium chloride, 4.2g. of pyridine (Py/Pd 6 times of fect of agitation of the reactor. In a batch-wise process, mols) and 150 g. of o-dichloro benzene as a, solvent suitable reaction is from 20 minutes to 4 hours. But, were charged. The conversion of 2,4-dinitro toluene shorter or longer reaction times do not adversely affect was 100% and the yield of toluene-2, 4-diisocyanate the reaction. I was 80.9% based on consumed dinitro toluene.

For the present invention, the reaction can be ef- 10 fected either batch-wise or continuously. EXAMPLE 3 Up to now, it is not clear in what forms the present Into a 120 ml. stainless steel (SUS 32) autoclave catalyst system with Py/Pd :4 mol works. Further, the there were charged 6.0 g. of nitro benzene, 0.5 g. of working mechanism is not clear. Assumingly, the comrhodium trichloride, 0.8 g. of pyridine and 30 g. of tri- ,plex of palladium.-pyridine-chlorine becomes in a most chloro benzene (the amount of rhodium based on nitro suitable state at the time of interaction of nitro combenzene is 5 mol and the amount of pyridine is 4 pound with carbon monoxide and cluring the comingtimes of mols based on rhodium), and then carbon off stage of isocyanate, thereby surpressing the formamonoxide with the pressure of 120 Kg/cm was filled. tion of other compound and increasing the yield of the The reaction was effected for 1.5 hours under stirring present product. by a shaking apparatus with oil bath maintained at Comparing to the known patents, the present inven- 200C (the amplitude being 20 cm; rotation being 40 tion gives higher yield of the isocyanate and the prodcycles per minute). The reaction liquid was treated and uct in a liquid detected by gaschromatography is solely analyzed as in Example 1. The conversion of nitro benisocyanate and as a consequence separation and purifizene was 100% and the yield of phenyl isocyanate was cation can besimplified. 74.5%.

Since no addition of phenol etc. is made in the resent invention, there is neither formation of carb mic EXAMPLE 4 acid nor corrosion of the reactor, The same procedure as in Example 3 was followed,

Metallic chlorides which greatly affect the producexcept that 1.1 g. of 2,4-dinitro toluene, 0.16 g. of pa]- tion cost of the isocyanate, form the complex with pyriladium dichloride, 0.28 g. of pyridine (Py/Pd 4 times dine. The complex can be recovered in a form of neeof mols) and 15 g. of benzene as a solvent were charged dle-like chrystal and can be reused by recycle. and the reaction was effected by using a shaking appa- In summary, the method according to the present inratus with oil bath maintained at 230C. The conversion vention has many advantages to enable the low cost of 2,4-dinitro toluene was 100% and the yield of tolproduction. of isocyanate and therefore can be emuene-2,4-diisocyanate was 73.5%. ployed as a commercially feasible process for producing the aromatic isocyanate in a large scale. EXAMPLE 5 8 Into a 120 ml. stainless steel (SUS 32) autoclave, EXAMPLE 1 there were charged 1.1 g. of 2,4-dinitro toluene, 0.16 g. Into a 500 ml. stainless steel (SUS 32) autoclave of palladium dichloride, 15.0 g. of o-dichloro benzene equipped with a magneticstirrer, there were charged and 4, 6, 9, or 20 times respectively of mols of pyridine 50.0 g. of nitro benzene, 1.9 g. of palladium dichloride, based on palladium, and then carbon monoxide with 3.4 g. of pyridine and 125 g. of monochloro benzene as the pressure of 120 Kg/cm at room temperature was a solvent and then carbon monoxide with the pressure filled. Then the reaction was effected by using a shakapproximately of 120 Kg/cm at room temperature was ing apparatus with oil bath maintained at 210C (the filled. The amount of palladium is 0.025 time of mol amplitude being 20 cm; rotation being 40 cycles per based on the starting compound and the amount of pyrminute). The reaction liquid was treated and analyzed idine is 4 times of mols based on palladium compound. as in Example 1. As a result, the conversion and the Under stirring at 800 1000 rpm by a magnetic stirrer, yield were as follows. The result was shown in the drawthe temperature was increased to 210C over a period of ing.

Pyridine/Palladium 2,4-dinitrotoluene Toluene-2,4-diisocyanate Example molar ratio Conversion (9?) Yield ("/2) 30 minutes and then by maintaining this temperature It can thus be seen from the drawing that when the the reaction was effected. Maximum gauge pressure molar ratio of pyridine to palladium ranges from 4 during the reaction was 200 Kg/cm After cooling the moles to 8 moles of pyridine per mole of palladium, the reactor to room temperature, the reaction liquid was yield of toluene-2,4-diisocyanate is in excess of 70%. taken out and the conversion of benzene and the yield 65 CONTROL 1 Into a ml. of stainless steel (SUS 32) autoclave, 1.1 g. of 2,4-dinitro toluene, 0.16 g. of palladium di chloride, 0.14 g. of pyridine and 15.0 g. of o-dichloro 7: benzene (the amount of palladium being mol based onr2,4-dinitro toluene; the amount of pyridine being 2 times of mols based on palladium), and then carbon monoxide with the pressure of 120 Kg/cm at room temperature was filled. The reaction was effected for 2.0 hours under shaking by using ashaking appara-' tus with oil bath maintained at 210C (the amplitude being cm; rotationbeing 4O cycles per minute).

After cooling the reactor to room temperature, the re-" CONTROL. 2

The same procedure andthe same reaction condition as in Control 1 were followed except that the amount of pyridine employed is equal molto that of palladium. The conversion of 2,4-dinitro toluene was 92.5% and the yield of toluene-2,4-diisocyanate was 34.0%. The result was shown in the drawing.

CONTROL 3 7 Into a 120 ml. of stainless steel (SUS 32) autoclave, there were charged 1.1 g. of 2,4-dinitro'toluene, 0.16 g. of palladium dichloride, 15.0 g. of o-dichloro benzene, 4.36 g. of phenol (phenol/2,4-dinitro toluene 7.72 times of mols) and 1, 2, 4, 6, or 9 times respectively of pyridine based on palladium, and then carbon monoxide with the pressure of 120 Kg/cm at room temperature was filled. The reaction was effected for 2.0 hours under shaking by using a shaking apparatus with oil bath maintained at 210C (the amplitude being 20 cm; rotation being 40 cycles per minute). The reaction liquid was treated and analyzed as in Control 1. As a result, the conversion and the yield of the reaction product were as follows. The result was shown in the draw- 40 8 room temperature was filled. The reaction was effected for 3.0 hours under shaking by using a shaking apparatus with oil bath maintained at 190C (the amplitude being'ZO cm; rotation being 40 cycles per minute).

After cooling the reactor to room temperature, the reaction'liquid was taken out and'theconversion of 2,4- dinitr o toluene and the'yield of the reaction product was calculated by gaschromatography analysis using biphenyl as an internal standard. The conversion was 92.5%, the yield of toluene-2,4-diisocyanate was and the yield of toluene-mononitro, monoisocyanate was 25.7%.

What is claimed isf l. Ina process of'preparing an aromatic isocyanate' byr'eacting an aromatic nitro compound with carbon monoxide'under pressure at'an elevated temperaturein the presence of a catalyst, the improvement comprising" conducting the reaction in the presence of an aromatic hydrocarbon or halogenated hydrocarbon solvent at a monoxide charged at room temperature of 40-200 kg/cm in the presence'of' a catalyst consisting of" a chloride of a metal selected from the group consisting of palladium and rhodium, and pyridine, the metal' chloride and pyridine being present either as a'mixtur'e or in the form'of a complex and the pyridine-being present in'an' amount ranging from- 4 moles to 8'moles' based on 1 mol of said metal element.

2. A process according 'to claim 1 wherein the reac-' tion is effected at a temperature of 200 240C.

3."A process according to claim 1, wherein the amount of palladium compound or rhodiumcompound is not less than 0.01% by weight based on aromatic nitro compound."

4. A process according to claim 1, wherein themg. monochloro' toluene, trichloroethane, 'tetrachloroeth- Pyridine/Palladium 2,4-dinitrotoluene Toluene 2,4-diisocyanate Control molar ratio Conversion (7;) Yield (f%) ane, trichlorotrifluoro ethane erchloroeth lene' and CONTROL 8 P y carbon monoxide with-the pressure of 120 Kg/cm at aromatic hydrocarbon such as benzene, toluene and xylene is used as a solvent.

6. A process according to claim 1, wherein aromatic nitro compound to be employed is monoand polynitro benzene and isomer mixture thereof; nitroalkyl benzene and isomer mixture thereof; and bis(nitropheny1) alkyl and isomer mixture thereof.

temperature of 200-250C under the pressure of carbon 

1. IN A PROCESS OF PREPARING AN AROMATIC ISOCYANTE BY REACTING AN AROMATIC NITRO COMPOUND WITH CARBON MONOXIDE UNDER PRESSURE AT AN ELEVATED TEMPERATURE IN THE PRESENCE OF A CATALYST THE IMPROVEMENT COMPRISING CONDUCTING THE REACTION IN THE PRESENCE OF AN AROMATIC HYDROCARBON OR HALOGENATED HYDROCARBON SOLVENT AT A TEMPERATURE OF 200*-250*C UNDER THE PRESSURE OF CARBON MONOXIDE CHARGED AT ROOM TEMPERATURE OF 40-200 KG/CM- IN THE PRESENCE OF A CATALYST CONSISTING OF A CHLORIDE OF A METAL SELECTED FROM THE GROUP CONSISTING OF PALLADIUM AND RHODIUM AND PYRIDINE THE METAL CHLORIDE AND PYRIDINE BEING PRESENT EITHER AS A MIXTURE OR IN THE FORM OF A COMPLEAND THE PYRIDINE BEING PRESENT IN AN AMOUNT RANGING FROM 4 MOLES TO 8 MOLES BASED ON 1 MOL OF SAID METAL ELEMENT.
 2. A process according to claim 1 wherein the reaction is effected at a temperature of 200* - 240*C.
 3. A process according to claim 1, wherein the amount of palladium compound or rhodium compound is not less than 0.01% by weight based on aromatic nitro compound.
 4. A process according to claim 1, wherein the amount of palladium compound or rhodium compound is 0.1 - 20% by weight based on aromatic nitro compound.
 5. A process according to claim 1, wherein one or mixture of monochloro benzene, dichloro benzene, monochloro toluene, trichloroethane, tetrachloroethane, trichlorotrifluoro ethane, perchloroethylene; and aromatic hydrocarbon such as benzene, toluene and xylene is used as a solvent.
 6. A process according to claim 1, wherein aromatic nitro compound to be employed is mono- and polynitro benzene and isomer mixture thereof; nitroalkyl benzene and isomer mixture thereof; and bis(nitrophenyl) alkyl and isomer mixture thereof. 